> there still seems to be a separation between music systems users and music systems developers.

That's true, but just like a pianist typically doesn't need to build their own piano, computer musicians rarely need to build their own DAWs or audio programming languages. However, computer musicians do build their own systems on top of Pd, SC, etc. and these can evolve into libraries or whole applications. So the line between computer musicians and audio application developers is blurry.

That being said, I can tell for sure that only few computer musicians end up contributing code to Pd, SC, etc., simply because most of them have no experience in other programming languages and are not really interested in actual software development. Of course, there are other important ways to contribute that are often overlooked, like being active on forums, filing bug reports, etc.